Low-pressure mercury vapour discharge tube



June 7, 1960 D. KOLKMAN 2,939,935

LOW-PRESSURE MERCURY VAFOUR DISCHARGE TUBE Filed Sept. 25, 1958 INVENTOR DIRK KOLKMAN AGET United States Patent Ofice Patented June 7, 1960 LOW-PRESSURE NIERCURY VAPOUR DISCHARGE TUBE Dirk Kolkman, Emmasingel, Eindhoven, Netherlands, as= signor to North American Philips Company Inc., New York, N.Y., a corporation of Delaware Filed Sept. 23, 1958, Ser. No. 762,768

Claims priority, application Netherlands Oct. 9, 1957 6 Claims. (Cl. 31'3-185) The invention relates to a low-pressure mercury vapour discharge tube having during operation a lowest external wall temperature of from 30 C. to 45 C. and a wall load over 50 mw. per sq. cm., and comprising at least one projection of the wall intermediate the electrodes, the radial size of which projection exceeds 20% of the radius of the tube.

This application is a continuation-in-part of application Serial No. 725,886.

This projection is a means of attaining said lowest wall temperature with the aforesaid load.

With this in regard o'f low-pressure mercury vapour discharge tubes considerable wall load there occurs strong disintegration of the activated thermionic electrodes, which involves severe blackening of the wall of the tube.

In normal discharge tubes, which are loaded to approximately 30 mw./sq. cm. wall surface, it is customary to counteract blackening by means of iron rings surrounding the thermionic electrodes. In the present tube, however, the use of this means did not yield satisfactory results.

The present invention has for its object to mitigate this disadvantage.

In accordance with the invention, each electrode is surrounded by a chrome-iron disintegration screen which is insulated from it. The screen preferably contains 15% to 35% by weight of chromium.

In order that the invention may be readily carried into effect, an example will now be described in detail with reference to the accompanying drawing, in which Figs. 1 and 2 show alternative embodiments of the invention.

The reference numeral 1 designates the cylindrical envelope of a low-pressure mercury vapour discharge tube. The envelope is closed at its ends, for example by means of conventional pinch structures 2 into which supply conductors '3, 3 for thermionic electrodes 4, activated with alkaline earth metal compounds, are sealed.

The envelope may internally be provided with a layer transforming the radiation produced in the discharge into radiation of greater wave length. Naturally, such a layer is omitted if the radiation produced in the discharge is to be used directly.

The middle portion of the envelope comprises a domeshaped projection 5 containing a small quantity, say mg. of mercury 6.

If desired, the projection may extend annularly about the cylindrical envelope, as indicated in dash-lines 7 in Fig. 2.

The thermionic electrodes are surrounded by chromeiron rings 8, a stay wire 9 of which is anchored in the pinch 2. Between the electrodes 4 and the ring 8 there is no electrically conductive connection, hence the potential of the rings is independent of that of the electrodes.

In a given case, the envelope was approximately 120 cms. long, inside diameter approximately 36 mms., outside diameter approximately 38 mms., spacing of an electrode from its associated tube end approximately 35 mms. The dome 5 projected approximately 10 mms. from the envelope and had a diameter of approximately 11 mms.

With a filling of argon at a pressure of approximately 0.8 mm. mercury at room temperature and a load adjusted by means of a ballast impedance of approximately 100 w., the tube voltage was approximately v. with a discharge current of approximately 1.45 amps., which corresponds to a wall load of approximately 74 mw. sq. cm, the temperature of the downwardly directed tip of the dome-shaped projection then being approximately 38 C.

A tube comprising a luminescent layer of halogen phosphate with a colour temperature of approximately 4200 C. had an efiiciency of approximately 61 lumen per Watt power input. Of course, the electrodes 4 were proportio'ned for said current strength.

The rings consisted of chrome-iron strip approximately 7 mms. wide and 0.1 mm. thick and surrounded the electrodes 4 with a spacing of a few millimetres. The material of the rings contained approximately 24% by weight of chromium.

When using these rings, a decrease in light of approximately 5% was measured after an operational period of hours, which is fairly normal.

When using conventional iron rings, however, the decrease in light was 15% to 20%.

What is claimed is:

l. A low-pressure mercury vapor discharge tube adapted to operate at a lowest outer wall temperature of about 30 C. to 45 C. and a wall load exceeding 50 milliwatts per cm. comprising a tubular envelope containing an ionizable medium, a pair of electrodes within said envelope, a supply of mercury disposed within at least one extension of the wall of the envelope which is disposed between the electrodes and the radial dimension of which exceeds 20% of the tube radius, and a chromium-iron disintegration screen surrounding and insulated from each of said electrodes.

2. A low-pressure mercury vapour discharge tube as claimed in claim 1, in which the screen contains about 15% to 35% by weight of chromium.

3. A low-pressure mercury vapor discharge tube as claimed in claim 1 in which the extension is annular and encircles the tube throughout its circumference.

4. A low pressure mercury vapor discharge tube as claimed in claim 1 in which the extension is dome-shaped.

5. A lo'w pressure mercury vapor discharge tube as claimed in claim 1 in which the ionizable medium consists of argon under a pressure of about 0.8 mm. mercury at room temperature.

6. A low-pressure mercury vapour discharge tube adapted to operate at a lowest outer wall temperature of about 30 C. to 45 C. and a wall load exceeding 50 milliwatts per cm. comprising a tubular light-transparent envelo'pe containing an ionizable medium, a pair of electrodes within said envelope, and a supply of mercury disposed within at least one extension of the wall of the envelope projecting outwardly from the tube and containing the mercury, which extension is disposed between the electrodes and the radial dimension of which exceeds 20% of the tube radius.

References Cited in the file of this patent UNITED STATES PATENTS 2,103,041 Ro'mpe et al Dec. 21, 1937 2,262,177 Germer Nov. 11, 1941 2,518,248 Ohlund Aug. 8, 1950 

